The present invention relates to a charging device for accumulating electric energy in a capacitor bank having a plurality of capacitors as power elements, and a charging method thereof.
Recently, a study has been conducted to accumulate electric energy in an electrical double layer capacitor and use the accumulated electric energy as a driving power source of an electric vehicle or the like. Meanwhile, a charging device for storing electric energy in a plurality of electrical double layer capacitors has been developed.
FIG. 19A is a block diagram showing a major portion of a conventional arrangement of such a charging device, and FIG. 19B is a view explaining an operation of the charging device.
As shown in FIG. 19A, the charging device comprises a power source circuit 110 for supplying a charging current, a capacitor bank 120 composed of a plurality of electrical double layer capacitors C11-C14 connected in series, and parallel monitor circuits 130A-130D respectively connected to the electrical double layer capacitors C11-C14, and the device is arranged in such a manner that, by supplying the capacitor bank 120 with a predetermined charging current IC from the power source circuit 110, charges corresponding to the charging current IC are accumulated in each of the electrical double layer capacitors C11-C14.
Generally, a voltage (charged voltage) V across a capacitor including the electrical double layer capacitor is expressed by the following equation: EQU V=Q/C (1)
where Q is a charge quantity and C is a capacity of the capacitor.
The charge quantity Q is expressed by the following equation: EQU Q=I.multidot.t (2)
where I is a current flowing through the capacitor, that is, a charging current, and t is a charging time.
Hence, the charged voltage V is expressed by the following equation: EQU V=(1/C).multidot.I.multidot.t (3).
That is, if the charging current I is constant, the charged voltage V increases as the charging time t extends. It should be noted that the voltage (charged voltage) V across the capacitor has a tolerance limit value (withstand proof voltage), and if the charged voltage V exceeds the withstand proof voltage of the capacitor, the capacitor is damaged or deteriorated, or the charging device breaks. Therefore, when charging the capacitor, the charging operation should be controlled so that the charged voltage will not exceed the withstand proof voltage of the capacitor.
For this reason, as shown in FIG. 19A, the conventional charging device is arranged in such a manner that the parallel monitor circuits 130A-130D are provided to the respective electrical double layer capacitors C11-C14, so that the parallel monitor circuits 130A-130D detect and monitor the terminal voltages of their respective capacitors C11-C14 as the charged voltages. In other words, as shown in FIG. 19B, the conventional charging device is arranged in such a manner that, let VL be the withstand proof voltage of the capacitors, then when the charged voltage in each capacitor exceeds the withstand proof voltage VL, the charging device bypasses the charging current IC to the parallel monitor circuits 130A-130D side, thereby stopping the charging operation for the electrical double layer capacitors.
In the foregoing conventional charging device, however, in order to charge a plurality of electrical double layer capacitors connected in series, a voltage monitor circuit for detecting and monitoring the charged voltage has to be connected to each electrical double layer capacitor in parallel. For this reason, the conventional charging device has a problem that its size undesirably increases as the number of the electrical double layer capacitors used therein increases, and so does the cost of production.
In addition, the conventional charging device is arranged in such a manner that, when the value of the charged voltage exceeds the predetermined withstand proof voltage, the charging current is bypassed to the parallel monitor circuits. Thus, a heat quantity is generated in response to the power consumption of the parallel monitor circuits. In other words, as shown in FIG. 19B, the heat quantity W generated in the charging device by bypassing the charging current is expressed by the following equation: EQU W=IC.multidot.VL.multidot.n (4)
where IC is the charging current, VL is the withstand proof voltage of the electrical double layer capacitors, and n is the number of the parallel monitor circuits. Hence, the heat quantity W generated by the voltage monitor circuits provided in the charging device increases in proportion to the number n of the voltage monitor circuits provided in the charging device. Also, the number of the voltage monitor circuits corresponds to the number of the electrical double layer capacitors used in the charging device. Therefore, the conventional charging device has a problem that a larger heat quantity is generated as the number of the electrical double layer capacitors used therein increases, and downsizing the charging device becomes more difficult as the number of the electrical double layer capacitors used therein increases.